Process for making small particles

ABSTRACT

A PROCESS FOR MAKING METALLIC PARTICLES OF SUB-MICRON PARTICLE SIZE WHEREIN A SOLUBLE METALLIC SALT IS REACTED WITH A STRONG REDUCING AGENT IN A SOLVENT MEDIUM WHICH, IN ADDITION TO THE AFORESAID REACTANTS, COMPRISES A SILANE SOLUTE WHICH HAS A STRONG CHEMICAL AFFINITY FOR THE NEWLYFORMED METALLIC SURFACE. THE INVENTION ALSO ENCOMPASSES PRODUCTS FORMED BY THE AFORESAID PROCESS, E.G. IRON AND COBALT POWDERS CHARACTERIZED BY A SUPRISINGLY HIGH BULK DENSITY WHEN COMPARED TO METAL PARTICLES PREPARED BY SOMEWHAT SIMILAR PROCESSES.

United States Patent O m 3,725,036 PROCESS FOR MAKING SMALL PARTICLESJohn E. Ehrreich, Wayland, and Adrian R. Reti, Cambridge, Mass.,assignors to Graham Magnetics, Inc., Graham, Tex. No Drawing. Filed Mar.24, 1971, Ser. No. 127,850 Int. Cl. B22f 9/00; H011? 1/06, N09

US. Cl. 75-.5 A Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THEINVENTION This invevntion relates to making small metallic particlescharacterized by extraordinary small particle size.

In recent years, a considerable amount of inventive effort has beenexpended in an attempt to obtain uniformly-shaped and sized, yet verysmall, particles of metallic substances in a readily dispersible form.Small metallic particles have a large number of uses. Their high surfacearea makes them useful in catalytic reactions utilizing the particularmetal. Such metal powders formed of magnetic metals can be utilized incompounding mixtures for use in making compressed powder orsinteredpowder magnets. Upon suitable after-treatment, such smallmagnetic particulate materials can be used, with a suitablesegregatingmatrix, in coating tapes for use in magnetic processes used ininformation storage and the like. Moreover, small metallic particles arealso useful in manufacture of pyrotechnic devices and can be consumed inhigh-energy fuel systems.

Some of the previous methods of making small particles have includedsuch processes as forming oxide particles and then reducing them byprocesses such as that disclosed by Robbins et al. in Journal of theElectrochemical Society, volume 117, page 137; or by electrodepositionof the metal as described in US. Pats. 3,073,- 762 and 3,198,716; or bydeposition of the metal from a metal carbonyl or the like. All of thesemethods are expensive, do not provide a means to control particle sizereadily-especiall in the sub micron range-and usually result inproduction of particles which have other than an optimum shape for aparticular use. Such processes tend to provide particles with arelatively high tendency to agglomerate with one another. The effect ofsuch agglomeration reduces the value of characteristics which would beexpected from the material on the basis of the basic particle sizethereof.

US. Pat. 3,206,338 to Miller and Oppegard contains a description of aprocess for forming very small metal particles by reducing a metal saltwith a metalborohydride while maintaining a magnetic field about thereaction zone. This process, although yielding very small uniformparticles, does not provide the degree of nonagglomeration sought by theinstant inventors. Thus the materials prepared thereby are rather fluffyand have relatively low bulk densities. Moreover, the process taught byMiller and Oppegard results in the production of excessively pyrophoricpowders when it is used to make powders of the more readily oxidizablemetals. It is also noted that there is a tendency for very smallparticles 3,725,036 Patented Apr. 3, 1973 formed byreduction-from-solution type processes to have an excessive oxidecontent and consequently have reduced reactivity.

SUMMARY OF THE INVENTION It is an object of the invention to provide anovel and improved means for making small, uniform, metallic particles.

Another object of the invention is to provide such metallic particles insubstantially discrete form as opposed to highly agglomerated form, andthereby preserve more perfectly those physical properties which areassociated with a mass of individual particles.

A further object of the invention is to provide a relatively simpleprocess for making such discrete particles which process does notrequire the use of highly expensive apparatus nor entail the use oftoxic gases.

A particular object of the invention is to provide means to controllablyinhibit the growth of metallic nuclei formed by the reduction of ametallic salt in a liquid medium.

A further object of the invention is to provide a means for making apyrophoric metallic powder of very small particle size.

Another object of the invention is to provide a small metal particlesusceptible to enhancement of its magnetic properties by thermal meanswithout undue agglomeration of the particles.

Another object of the invention is to provide metal powders ofcontrolled pyrophoricity.

Other objects of the invention will be obvious to those skilled in theart on reading the instant application.

The above objects have been substantially achieved by the reduction ofmetallic salts in the presence of a metal-surface passivating agent.This reduction is carried out in a liquid phase in which both themetallic salt and passivating agent have substantial solubility. Theproducts are free-flowing powders having a bulk density of about amagnitude greater than materials prepared by analogous processes butwithout silane.

By the designation of silane as a passivating agent, it is not meant toimply that the particular passivating agent performs its passivatingfunction without any significant chemical change. Indeed the majority ofcompounds suitable for use as passivating agents will first behydrolized, or otherwise modified by interaction within the reactionmedium, before producing an activity-passivating moiety, Le. a moietyhaving the suitable electronic character to seek out and neutralize theactive growth sites on the metallic particles being formed.

The reducing agent which is used can be any strong reducing agentpermitting the metal salt to be reduced to the metal itself. Metalborohydrides are particularly useful in aqueous media at moderatetemperatures. Among such metal borohydrides are the alkali metalborohydrides such as sodium and potassium borohydrides, the alkalineearth borohyidrides such as calcium borohydride, magnesium borohydride,and the like.

The precise particle size of the metallic particles being formed will,in part, depend on the concentration, temperature, and degree ofagitation of the fluid mass in which the reduction is accomplished.However, the particle size is also influenced by the choice of a properpassivating agent which must be characterized by a moderate activity insolution whereby it does not react so quickly with newly-formed metalthat one obtains little more than a organometallic compound, or metallicoxide dust, yet does combination quickly enough with active-sites on anewlyforming metal surface to prevent the growth of metal particles ofexcessive size.

The particles formed by the process described herein have the surprisingcharacteristic of becoming more magnetic rather than less magnetic Whensubjected to elevated temperatures, eg, in the 300650 range in anon-oxidizing atmosphere.

Silanes, especially those having molecular Weights of from about 100 to500, are particularly desirable passivating agents when one wishes toproduce, at moderate temperatures, dispersions of metallic particles inthe submicron range, for example those having maximum average diametersbelow 0.5 micron.

As pointed out in U.S. Pat. 3,206,338, reduction processses of thegeneral type described herein may be carried out within a strongmagnetic field. It has now been discovered that, when no magnetic fieldis involved, and when the particular process improvements describedherein are utilized, particles of surprisingly little magnetic activitycan be manufactured. This is believed to be a consequence of the use ofthe passivating agent to inhibit growth of the metal particles. However,often it will be desirable to operate within a magnetic field (or,alternatively, posttreat the particles with heat) to achieve desiredmagnetic properties.

The precise mechanism by which the passivating agent operates is notprecisely known. Indeed different passivating agents may achieve theirresults by different chemical and physical effects within the reactionsystem. Without any intention of being bound by the theory, it isbelieved that the passivating agent provides a moderately reactivechemical group, such as the hydrolysis product of a silane molecule,which group tends to seek a site on a surface of the growing metalparticle which site is, if only fieetingly, of the oppositeelectro-negative charge from the aforesaid hydrolysis product. Thepassivating group attaches itself to the metal particle and, either bydirectly pre-empting the position at which a metal ion would havedespited or by a steric hindrance with such a site, tends to stop thegrowth of the metal particle at an early stage in its growth.

In any event, the passivating agent must be sufliciently active tocomplete its inhibiting action before the metal particles growexcessively in size. For this reason, among others, macromolecularpassivating agents do not seem to be generally effective in manufactureof metal particles of very small size.

Metallic salts may be selected from a Wide range of materials. Theefiicacy of the process is limited, as will be generally understood, tothose metals susceptible to reduction by hydrogen. The more important ofthese are iron and cobalt. The more convenient salts are the commoncommercial grades of sulfates, nitrates, halides such as bromides andchlorides, salts of organic acids, and the like. It is not believed anyspecial advantage can be achieved by use of more expensive metal salts.

It is often advantageous to heat treat particles produced by the abovedescribed process in order to modify the basic properties thereof. Ofcourse, this heat treatment must be carried out by a means which willlargely preserve the special attribute of the particulate product, i.e.a surprisingly low degree of agglomeration and interaction betweendiscrete particles.

Very fine particles of refractory materials, i.e. materials having amelting or decomposition points above the point at which the metalparticles undergo significant physical modification, can be mixed intothe metal particles prior to the heat-treating step. This type of mixinghas been found to minimize any agglomeration of the metal particlesduring heat treatment.

The particles mixed into the metal particles must be separable therefromby some distinct physical or chemical characteristic such as solubilityor density. In practice, this presents no diificulty since such commonlyavailable highly soluble refractory materials such as sodium chloridemay be readily leached out of admixture with metal powders. Manyrefractory materials, although non-soluble, can be readily separatedfrom metallic particles by known centrifugal techniques.

The process using silane passivating agents according to the inventionallows production of powders having unusually small particles sizes,below 1,000 angstroms and even below angstroms, and excellent size andshape uniformity. However, it also provides a means for making smallmetallic powders of reduced pyrophoricity and means for providingrelatively dense metal powders having unusually high apparent densitiesin relation to their very small particle sizes. Sub micron particleswith apparent bulk densities of 0.3 to 0.8 gm./cc. can be prepared(using iron as a standard for such a measurementcobalt would differbecause of its differing density only).

The pyrophoric nature of the more-readily oxidizible metals can beenhanced by working with such silane-attacking solvents astetrahydrofuran or the like.

On advantage achieved by use of a passivating agent is to enable theproduction of a small metallic powder which, despite its small size, canbe readily converted to a pyrophoric form by washing with a suitablepassivatorremoving substance. For example, a silane-bearing material canbe washed with tetrahydrofuran or the like to produce a pyrophoricpowder having an extraordinary low particle size. Powders having suchparticle sizes have ordinarily had such a thick protective layer of anoxide or have had so little real metallic content that they have notbeen pyrophoric and therefore have not been efficacious for compoundinginto pyrotechnic compositions, for use in high-energy fuel systems, orlike applications requiring a significant quantity of quickly-oxidizablemetal surface.

When the process of the invention is carried out within a magneticfield, the force of the field should be at least about 500 oersteds tocontribute any important magnetic properties. Moreover, it is desirableto use a passivating agent of relatively low activity in the saltsolution so that somewhat larger particles will be formed during theprocess.

Illustrative Examples of the Invention In order to point out more fullythe nature of the present invention, the specific embodiments of thepresent process and products produced thereby are set forth below:

WORKING EXAMPLE 1 A 0.5 molar, aqueous solution of cobalt chloride wasprepared. A quantity of 25 drops of a vinyl silane,vinyltriacetoxysilane having a molecular weight of 232 and sold underthe trade designation Z6075 by Dow Corning Corporation, was added to 100milliliters of this aqueous solution using a conventional eye dropper.

In a separate vessel 2.0 grams of sodium borohydride was dissolved in 50milliliters of water.

Thereupon, using a magnetically-actuated mixing bar to maintain adequateagitation, the borohydride solution was added dropwise to the cobaltchloride solution. Care was taken to add the borohydride slowly enoughto allow the substantially complete reaction of the borohydride in eachdrop before adding a subsequent drop.

The reaction resulted in a very fine dispersion of cobalt which wasrelatively free from agglomeration of individual metal particles.

When the same reaction was run without the use of the silane a highlyconglomerated cobalt dispersion was obtained which settled rapidlyrather than remained in dispersion.

WORKING EXAMPLE 2 The procedure of Example 1 was followed excepting thatan amino silane, n-beta aminoethyl-gamma-aminopropyl trimethoxysilane,having a M.W. of 222 and sold under the trade designation Z6020 by DowCorning Corporation, was used instead of the vinyl silane of Example 1.

The resulting cobalt particles were similarly dispersed to those made inpresence of the vinyl silane. The dispersion was clearly distinguishableas having relatively little agglomeration as compared to the dispersionprepared without using a passivating agent.

WORKING EXAMPLE 3 The procedure of Example 2 was followed excepting theamino silane was added to the borohydride mixture instead of the cobaltsalt. The product was similar to that obtained in Example 2.

WORKING EXAMPLE 4 The procedure of Example 3 was followed excepting thatonly five drops of amino silane were added to the borohydride solution.

The resulting dispersion of cobalt particles was filtered, washedseveral times with water and air dried at about 30 F.

The resulting cobalt powder was extremely fine and in a substantiallynon-agglomerated form.

WORKING EXAMPLE 5 Two grams of amino silane was added to a solution ofgrams sodium borohydride in 100 milliliters of water. This borohydridesolution was added slowly to 340 milliliters of a one molar cobaltchloride solution as this latter solution was agitated in a WaringBlendor.

The resulting precipitate of cobalt metal was washed with water severaltimes, then washed with tetrahydrofuran. A magnet was used to separatethe powder from the tetrahydrofuran.

Thereupon the material was dried in a vacuum oven. The resulting, veryfine, relatively non-agglomerated material was relatively pyrophoriccompared to those materials prepared according to the first four workingexamples.

WORKING EXAMPLE 6 A quantity of 9.9 grams of FeCl -4H 0 was dissolved in100 milliliters of water.

Two grams of sodium borohydride and five drops of amino silane wasdissolved in 50 milliliters of water in a separate vessel.

The iron-bearing solution was agitated with a magnetic stirrer and theborohydride solution was slowly added thereto. A fine dispersion of ironpowder was obtained.

After being filtered and washed four times with distilled water, theiron powder was dispersed in acetone to aid the drying thereof. Thepowder was then separated from acetone by use of a magnet and air driedat 30 F.

When the foregoing procedure was followed without use of a passivatingagent, an iron powder was obtained which had agglomerated into a flufiymaterial of relatively low apparent density.

The bulk density of the material prepared with the passivating agent wascompared to the bulk density of the material prepared without thepassivating agent:

With: 0.53 gm./cc. Without: 0.06 gm./cc.

WORKING EXAMPLE 7 The general procedure described in Example 1 iscarried out in a magnetic field of about 1500 oersteds, i.e. in a smallglass vessel placed between the poles of a permanent magnet whichcontributes a nominal field strength of about 5000 oersteds.

One gram of vinyl silane is used as a passivating agent.

The resulting material is magnetic, and relatively nonpyrophoric.

It is of course to be understood that the foregoing examples areintended to be illustrative and that numerous changes can be made in thereactant proportions, and conditions set forth therein without departingfrom the spirit of the invention as defined in the appended claims.

What is claimed is:

1. In a process for making small metallic particles, in solution, byreaction of a reducing agent with a salt of a metal, the improvementcomprising the steps of (at) introducing a passivating agent into saidsolution;

(b) maintaining a quantity of said passivating agent in said solutioneffective to inhibit growth of metal particles until said reducingreaction is completed.

2. A process as defined in claim 1 wherein said reducing agent is ametal borohydride.

3. A process as defined in claim 1 wherein said metal salt is awater-soluble salt of a ferromagnetic metal, and wherein said solutionis an aqueous solution.

4. A process as defined in claim 1 wherein said passivating agent ischaracterized by formation in solution of a radical having an electronicattraction for newly formed surfaces of said metal particles.

5. A process as defined in claim 1 wherein said passivating agent is asilane.

6. A process as defined in claim 5 wherein said passivating agent is awater-soluble silane.

7. A process as defined in claim 1 comprising the further step ofremoving said passivating agent from said metal particle, therebyproviding a pyrophon'c powder.

8. A process as defined in claim 1 wherein said process is carried outin a magnetic field of above 500 oersteds.

9. A process as defined in claim 1 comprising the additional step ofheat treating said metal in a non-oxidizing atmosphere as a means toenhance the magnetic properties thereof.

10. A process for preventing the agglomeration of very small metallicparticles and providing a mass of such particles in a form exhibiting avery high ratio of surface area and yet maintaining a relatively highbulk density, said process comprising the steps of (a) reducing asolution of metal salts with a metallic borohydride solution as areducing agent,

(b) simultaneously maintaining a quantity of a passivating agentdissolved in the reaction medium, and

(c) recovering metallic particles formed by the reducing reaction.

11. A process as defined in claim 10 wherein said passivating agentforms an oxidation-resistant protective barrier on said particle,thereby contributing oxidationresistance to the metallic particles.

12. A process as defined in claim 10 wherein said passivating agent is asilane.

13. A process as defined in claim 12, comprising the additional step ofremoving said silane from the surface of said metal, leaving apyrophoric metal composition.

14. A process as defined in claim 10 wherein said metal is cobalt, ironor a mixture thereof.

15. A process as defined in claim 13 wherein said metal is cobalt, ironor a mixture thereof.

References Cited UNITED STATES PATENTS 3,661,556 5/1972 .lolley et a1.0.5 AA

WAYLAND W. QIALLARD, Primary Examiner US. Cl. X.R.

750.5 AA, 0.5 AB; 148-105

